Evaluate the efficacy of retinoids of differing chemical structures to prevent the development of colon cancer during its preneoplastic period. Specifically: utilize a rat model whereby early lesions of epithelial cancers will be recognized and used to test the efficacy of retinoids in the prevention of preneoplastic lesions; study the effect of synthetic retinoids on the induction of colon cancer using the direct-acting carcinogen N-methyl-N-nitrosourea (MNU) and the indirect-acting carcinogen 1,2-dimethylhydrazine (DMH); study the time of action and effect of dose by administering MNU and DMH either as a single dose (or two doses for MNH) or in multiple doses detect early lesions of colon cancer and evaluate the effect of retinoids in their modification.